Thermoelectric Nanocomposite Materials

ABSTRACT

Thermoelectric (TE) nanocomposite material that includes at least one component consisting of nanocrystals. A TE nanocomposite material in accordance with the present invention can include, but is not limited to, multiple nanocrystalline structures, nanocrystal networks or partial networks, or multi-component materials, with some components forming connected interpenetrating networks including nanocrystalline networks. The TE nanocomposite material can be in the form of a bulk solid having semiconductor nanocrystallites that form an electrically conductive network within the material. In other embodiments, the TE nanocomposite material can be a nanocomposite thermoelectric material having one network of p-type or n-type semiconductor domains and a low thermal conductivity semiconductor or dielectric network or domains separating the p-type or n-type domains that provides efficient phonon scattering to reduce thermal conductivity while maintaining the electrical properties of the p-type or n-type semiconductor.

CROSS-REFERENCE

This Application is a Nonprovisional of and claims the benefit of priority under 35 U.S.C. § 119 based on U.S. Provisional Pat. Application No. 63/264,857 filed on Dec. 3, 2021. The Provisional Application and all references cited herein are hereby incorporated by reference into the present disclosure in their entirety.

FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

The United States Government has ownership rights in this invention. Licensing inquiries may be directed to Office of Technology Transfer, US Naval Research Laboratory, Code 1004, Washington, DC 20375, USA; +1.202.767.7230; techtran@nrl.navy.mil, referencing Navy Case # 210910.

TECHNICAL FIELD

The invention is related to a nanostructured thermoelectric material, more particularly to a method for making a thermoelectric nanocomposite material which can be p- or n-type.

BACKGROUND

Practical applications for thermoelectric (TE) materials increase dramatically when such materials have a figure of merit (ZT) above 2. The simplified dimensionless figure of merit for a TE material is quantified by the equation ZT =

$\frac{\sigma S^{2}T}{\kappa},$

where _(σ) is the electrical conductivity of the material, S is its Seebeck coefficient, T is its temperature, and _(K) represents the total electronic and lattice contributions (_(Ke) + _(KL)) to the material’s thermal conductivity. To date, much of the experimental work in this field has aimed to increase a material’s ZT by decreasing its thermal conductivity through incorporation of periodic nanoscale structures that reduce _(K) via phonon scattering and enhance S via selective scattering of lower-energy charge carriers.

However, efforts to improve the ZT of TE materials have been stagnant for decades. Modifications to ZT are difficult largely due to the intrinsic inverse relationship between _(K) and _(σ).

Recently, researchers have demonstrated a ZT greater than 2 in a select class of materials consisting of hierarchical microstructures with nano-sized precipitate phases which serve to effectively retard thermal transport without dramatic decay of electrical transport. Although such materials are fabricated with an approach that creatively exploits metallurgical principles in materials science, this approach cannot effectively control or fine-tune the composition, size and phase identities of the nanocrystalline domains.

As such, much of the “high ZT” literature demonstrations are one-shot experiments that explore the properties of a single material with one particular microstructure. Consequently, universal relationships between the performance of TE materials and their nanostructured design are not yet fully understood, and, therefore, cannot be optimized for specific applications.

In general, the thermal conductivity of a given material drops rapidly as its internal microstructural dimensions are reduced. Additionally, interfaces within nanostructured materials - particularly nanocomposites - can serve as energy-dependent scattering sites (“energy filters”) that enhance the mobility differential between hot and cold electrons, thereby increasing S. Unfortunately, electrical conductivity of a TE material is often reduced as its thermal conductivity decreases, preventing a useful increase in the material’s ZT. On the other hand, in truly nanostructured solids, electrical coupling between different nanostructures (or grains) is determined by the properties of the “linkages” (or grain boundaries) and interfacial structure specifics (coherence, orientation, etc.), which may lead to the formation of three-dimensional conduction bands within the material. Also, shortening of the distances between proximate nanostructures in the material increases the overlap integral between electron wave functions, thus increasing the conductivity. Therefore, the possibility of improving electrical conductivity — and subsequently ZT — through novel approaches to nanostructuring of TE materials is a viable option for boosting their figure of merit.

One approach that has been used to form nanocomposite materials uses the environmentally controlled pressure-assisted sintering (EC-PAS) methodology developed by researchers at the U.S. Naval Research Laboratory, including inventors of the present invention, aspects of which are illustrated by the block schematics in FIGS. 1A and 1B. See U.S. Pat. 10,497,530 to Feigelson et al., entitled “Thermionic Tungsten/Scandate Cathodes and Methods of Making the Same”; U.S. Pat. 10,513,462 to Feigelson et al., entitled “Transparent Nanocomposite Ceramics Built From Core/Shell Nano Particles”; U.S. Pat. 10,751,801 to Feigelson et al., entitled “Bulk Monolithic Nano-Heterostructures and Method of Making The Same”; and U.S. Pat. 11,075,049 to Feigelson et al., entitled “Thermionic Tungsten/Scandate Cathodes and Methods of Making the Same.” See also U.S. Pat. Application Nos. 17/951,168 and 17/951,181 by Feigelson et al., entitled “Bipolar Nanocomposite Semiconductors.”

As illustrated in FIGS. 1A and 1B, a nanocomposite material can be formed from a starting nanoparticle powder which is cleaned to form an “active” pristine surface. As illustrated in FIG. 1A, in some cases, the particles remain uncoated, while in other cases, such as the case illustrated in FIG. 1B, the cleaned nanoparticles can be coated with another material, e.g., by means of atomic layer deposition (ALD) to form core-shell nanoparticles. In either case, a porous preform is made from the nanoparticles. The porous preform is then sintered at a specified pressure and homologous temperature, e.g., a temperature less than 40% of the absolute melting point T_(m) in Kelvin or Rankine, to produce a solid that retains the nanostructure of the original nanoparticles but loses the porosity of the preform.

SUMMARY

This summary is intended to introduce, in simplified form, a selection of concepts that are further described in the Detailed Description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. Instead, it is merely presented as a brief overview of the subject matter described and claimed herein.

The present invention provides a thermoelectric (TE) nanocomposite material in which control of its charge transport is spatially decoupled from control of its heat transport. By doing so, the present invention enables design of a TE nanocomposite material that combines efficient electronic transport and suppressed thermal transport associated with different nanocomposite constituents.

The present invention provides a TE nanocomposite material that includes at least one component consisting of nanocrystals. A TE nanocomposite material in accordance with the present invention comprises a three-dimensional nanoparticle material network of bonded nanoparticles of a first material having a p- or n-type conductivity embedded within a solid comprising a second material having a thermal conductivity lower than a thermal conductivity of the first material, wherein the nanoparticle material network of the first material retains its nanostructure within the solid material, a p- or n-type network formed by the first material percolates charge through the entire TE material; and the second material provides a level of phonon scattering in the TE nanocomposite material so as to reduce its thermal conductivity while maintaining electrical transport properties provided by the percolating p- or n-type network formed by the first material

A TE nanocomposite material in accordance with the present invention can include, but is not limited to, multiple nanocrystalline structures, nanocrystal networks or partial networks, or multi-component materials, with some components forming connected interpenetrating networks including nanocrystalline networks.

In some embodiments, a TE nanocomposite material in accordance with the present invention can be in the form of a thermoelectric composite comprising a bulk solid having semiconductor nanocrystallites as part of the material, where the semiconductor nanocrystallites form an electrically conductive network within the material.

In some embodiments, a TE nanocomposite material in accordance with the present invention can be in the form of a nanocomposite thermoelectric material having one network of p-type or n-type semiconductor domains and a low thermal conductivity semiconductor or dielectric network or domains separating the p-type or n-type domains, where this highly resistive network provides efficient phonon scattering that reduces thermal conductivity and heat transport in the thermoelectric nanocomposite while maintaining the electronic transport in the p-type or n-type semiconductor network, with at least one of the networks comprising 3D, 2D, or 1D nanocrystals/nanocrystallites or possessing at least one nanoscale dimension.

In some embodiments, a TE nanocomposite material in accordance with the present invention comprises a thermoelectric nanocomposite having one network of p- or n-type semiconductor domains and another network of insulator/dielectric domains, with at least one of the networks consisting of 3D, 2D, or 1D nanocrystals/nanocrystallites or having at least one nanoscale dimension.

In some embodiments, a TE nanocomposite material in accordance with the present invention comprises at least one network that includes areas of still another material.

In some embodiments, the nanocrystals/nanocrystallites used in a TE nanocomposite material in accordance with the present invention can range in dimensions from 1 nm to 800 nm.

In some embodiments, at least one nanoscale length dimension of the nanoscale constituents used in a TE nanocomposite material in accordance with the present invention can have this dimension in the range of 1 nm to 800 nm.

In some embodiments, at least one nanoscale length dimension of the constituents used in a TE nanocomposite material in accordance with the present invention can have this dimension less than 1 nm.

In some embodiments, a TE nanocomposite material in accordance with the present invention can comprise strongly electronically coupled nanoscale networks with p- or n-type conductivity.

TE nanocomposite materials in accordance with the present invention include interpenetrating networks of p- or n-type semiconductor domains and at least one another component, typically an insulator or dielectric, which provides efficient phonon scattering behaving as a thermal barrier. Semiconductors responsible for charge transport in TE nanocomposite materials that can be used in accordance with the present invention include, but are not limited to, chalcogenides and their alloys, simple and compound semiconductors and their alloys, and compositions such as SnSe, Bi₂Te₃, Bi-Te alloys, BiSbTe alloys, Bi₂T₃/CdTe core/shells, Zn-Sb alloys, Si, Ge, SiGe, Mg₂Si, SrTiO₃, NaCo₂O₄, Zn₄Sb₃, Co-Sb alloys, and ZnO, while the insulator/dielectrics responsible for reduced thermal transport include insulators or dielectrics such as carbides, oxides, nitrides, fluorides, silicides, phosphides, sulfides, chlorides, and their alloys, including but not limited to SiC, A1₂O₃, ZrO₂, HfO₂, SiO₂, Gd₂Zr₂O₇, and (Zr,Hf)₃Y₃O₁₂, Si₃N₄, A1N, ScN, MgF, CaF, ZnF, A1P, SiS₂, LiCl, NaCl, MgCl₂, CaCl₂.

These and other aspects of the invention will be described in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are block schematics illustrating aspects of environmentally controlled pressure-assisted sintering (EC-PAS) formation of materials from nanoparticles in accordance with the prior art.

FIG. 2 is a block schematic illustrating aspects of an exemplary process flow for forming a thermoelectric (TE) nanocomposite material in accordance with one or more embodiments of the present invention.

FIGS. 3A-3D are block schematics illustrating aspects of forming, infilling and sintering a porous compact of nanoparticles to form a TE nanocomposite material in accordance with one or more embodiments of the present invention.

FIGS. 4A-4D are block schematics illustrating aspects of forming, infilling and sintering a porous compact of core/shell nanoparticles to form a TE nanocomposite material in accordance with one or more other embodiments of the present invention.

FIGS. 5A-5D are block schematics illustrating aspects of forming and sintering a porous compact of core/shell nanoparticles to form a TE nanocomposite material in accordance with one or more embodiments of the present invention.

FIGS. 6A-6D are block schematics are block schematics illustrating three-particle fragments of exemplary porous compacts after infilling but before sintering according to various embodiments of the methods for making a TE nanocomposite material in accordance with one or more embodiments of the present invention.

FIG. 7 is a plot illustrating the advantageous performance of TE materials formed in accordance with the present invention.

DETAILED DESCRIPTION

The aspects and features of the present invention summarized above can be embodied in various forms. The following description shows, by way of illustration, combinations and configurations in which the aspects and features can be put into practice. It is understood that the described aspects, features, and/or embodiments are merely examples, and that one skilled in the art may utilize other aspects, features, and/or embodiments or make structural and functional modifications without departing from the scope of the present disclosure.

The present invention provides a thermoelectric (TE) nanocomposite material in which control of its charge transport is spatially decoupled from control of its heat transport. By doing so, the present invention enables design of a TE nanocomposite material that combines efficient electronic transport and suppressed thermal transport associated with different nanocomposite constituents.

The present invention provides a TE nanocomposite material that includes at least one component consisting of nanocrystals. A TE nanocomposite material in accordance with the present invention comprises a three-dimensional nanoparticle material network of bonded nanoparticles of a first material having a p- or n-type conductivity embedded within a solid comprising a second material having a thermal conductivity lower than a thermal conductivity of the first material, wherein the nanoparticle material network of the first material retains its nanostructure within the solid material, a p- or n-type network formed by the first material percolates charge through the entire TE material; and the second material provides a level of phonon scattering in the TE nanocomposite material so as to reduce its thermal conductivity while maintaining electrical transport properties provided by the percolating p- or n-type network formed by the first material

A TE nanocomposite material in accordance with the present invention can include, but is not limited to, multiple nanocrystalline structures, nanocrystal networks or partial networks, or multi-component materials, with some components forming connected or percolated interpenetrating networks including non-crystalline and nanocrystalline networks.

In some embodiments, a TE nanocomposite material in accordance with the present invention can be in the form of a thermoelectric composite comprising a bulk solid having semiconductor as part of the material, where the semiconductor forms an electrically conductive network within the material.

In some embodiments, a TE nanocomposite material in accordance with the present invention can be in the form of a nanocomposite thermoelectric material having one network of p-type or n-type semiconductor domains and a low thermal conductivity semiconductor or dielectric network or domains separating the p-type or n-type domains, where this low thermal conductivity network provides efficient phonon scattering that reduces thermal conductivity and heat transport in the thermoelectric nanocomposite while maintaining the electronic transport in the p-type or n-type semiconductor network, with at least one of the networks comprising 3D, 2D, or 1D nanocrystals/nanocrystallites or possessing at least one nanoscale dimension.

In some embodiments, a TE nanocomposite material in accordance with the present invention comprises a thermoelectric nanocomposite having one network of p- or n-type semiconductor domains and another network of insulator/dielectric domains, with at least one of the networks consisting of 3D, 2D, or 1D nanocrystals/nanocrystallites or having at least one nanoscale dimension.

In some embodiments, a TE nanocomposite material in accordance with the present invention comprises at least one network that includes areas of still another material.

In some embodiments, the nanocrystals/nanocrystallites used in a TE nanocomposite material in accordance with the present invention can range in dimensions from 1 nm to 800 nm.

In some embodiments, at least one nanoscale length dimension of the nanoscale constituents used in a TE nanocomposite material in accordance with the present invention can have this dimension in the range of 1 nm to 800 nm.

In some embodiments, at least one nanoscale length dimension of the constituents used in a TE nanocomposite material in accordance with the present invention can have this dimension less than 1 nm.

In some embodiments, a TE nanocomposite material in accordance with the present invention can comprise strongly electronically coupled nanoscale networks with p- or n-type conductivity.

TE nanocomposite materials in accordance with the present invention include interpenetrating networks of p- or n-type semiconductor domains and at least one another component, typically an insulator or dielectric, which provides efficient phonon scattering behaving as a thermal barrier. Semiconductors responsible for charge transport in TE nanocomposite materials that can be used in accordance with the present invention include, but are not limited to, chalcogenides and their alloys, simple and compound semiconductors and their alloys, and compositions such as SnSe, Bi₂Te₃, Bi-Te alloys, BiSbTe alloys, Bi₂T₃/CdTe core/shells, Zn-Sb alloys, Si, Ge, SiGe, Mg₂Si, SrTiO₃, NaCo₂O₄, Zn₄Sb₃, Co-Sb alloys, and ZnO, while the insulator/dielectrics responsible for reduced thermal transport include insulators or dielectrics such as carbides, oxides, nitrides, fluorides, silicides, phosphides, sulfides, chlorides, and their alloys, including but not limited to SiC, A1₂O₃, ZrO₂, HfO₂, SiO₂, Gd₂Zr₂O₇, and (Zr,Hf)₃Y₃O₁₂, Si₃N₄, A1N, ScN, MgF, CaF, ZnF, A1P, SiS₂, LiCl, NaCl, MgC1₂, CaC1₂. It will be noted that one skilled in the art will readily recognize that the listed materials are exemplary only, and that other suitable materials can be used, and TE nanocomposite materials made from such other suitable materials are deemed to be within the scope of the present invention.

These and other aspects of this invention can be accomplished by the new process of making a thermoelectric nanocomposite described in detail in the disclosure of this invention.

The TE nanocomposite materials of the present invention can be made by the processes described below. In accordance with the present invention, these processes include several steps described herein, with each step of the process being a preferred part, and all steps taken together make the process sufficient to produce a TE nanocomposite material having the desired properties. To provide clean interfaces between all constituents of the synthesized TE nanocomposite material, all processing steps are preferred to be conducted in a controlled atmosphere and with air-free transfer between steps.

It should be noted, however, that the described processes are merely exemplary, and that other suitable processes for making a TE nanocomposite material in accordance with the present invention can be used, and all suitable processes and TE nanocomposite materials made from such processes are deemed to be within the scope of the present invention.

The flow diagram in FIG. 2 and the block schematics in FIGS. 3A-3D, 4A-4D, 5A-5D, and 6A-6D illustrate aspects of the process steps used in various exemplary embodiments of processes for making a TE nanocomposite material in accordance with the present invention. The basic process steps in each of the embodiments described below are the same, and so will not be repeated in the description of each embodiment for the sake of brevity, with only the process steps that are different from those in other embodiments being described in detail. For example, in the description of the second embodiment of a method for making a TE nanocomposite material in accordance with the present invention, Steps I, II, and IV proceed as described with respect to the first embodiment, with only Step III being described in detail.

Thus, as shown as Step 201 in FIG. 2 and as schematically illustrated in FIG. 3A, Step I in an exemplary first embodiment of this process includes selecting or making a powder consisting of particles or/and nanoparticles of a p- or n-type semiconductor material X₁, which mainly controls the electronic transport properties of a TE material. The powder of the semiconductor material can be made by any suitable technique such as ball milling, laser ablation, precipitation from solution, or hydrothermal and/or ammonothermal synthesis. In many embodiments, the powder will be a nanopowder comprising nanoparticles having a particle size of about 1 nm to about 800 nm. In some embodiments a particle size can be larger than 800 nm.

In an optional step 201 a, adsorbates such as water or oxides from the powder surface can also be removed in this Step I before proceeding to the next steps. This surface cleaning step can be accomplished by any suitable technique, though it is preferred that the cleaning is conducted in a furnace at elevated temperatures in a controlled atmosphere. Oxide removal can be conducted chemically or at elevated temperatures by reduction in an atmosphere with hydrogen.

In Step II of the process for making a TE nanocomposite material in accordance with the present invention, shown as Step 202 in FIG. 2 and in FIG. 3B, the powder of material X₁ is formed into a porous compact, creating a particle or nanoparticle interconnected network having an open porosity that permits a gas or a liquid to permeate the compact.

The porous compact made in this Step can be made by any existing technique such as techniques involving assembling or growing blocks of a porous compact or techniques making pores in existing material. In some cases, the compact can be made by pressing the powder of material X₁ in the container or die to form what is usually called a green compact. In other embodiments, the porous compact can be made by sintering the loose nanoparticle powder such that particles neck without densification and form strong chemical or metallic bonds with each other.

In an optional step 202 a shown in FIG. 2 , the green porous compact of material X₁ can be partially sintered while preserving an open porosity and making strong chemical or metallic bonds between particles.

In Step III of the process for making a TE nanocomposite material in accordance with the present invention, shown as Step 203 in FIG. 2 and schematically in FIG. 3C, the porous compact of material X₁, which initially has a form similar to that shown by the schematic in FIG. 3B, is infilled with a second material which is intended to minimize thermal transport in the TE material by increasing phonon scattering. In some embodiments, this second material Y₁ can be the same semiconductor material as X₁, while in other embodiments, the second material Y₁ can be a different semiconductor material or an insulator/dielectric material, as shown in as shown in FIG. 3C and FIG. 6A, so long as this second material Y₁ provides efficient phonon scattering in the final TE nanocomposite material so as to reduce its thermal conductivity while maintaining or improving electrical transport properties provided by p- or n-type material X₁.

During this infilling step, a continuous or discontinuous conformal layer of the second material Y₁ i is applied to partially or completely coat all available surfaces in the pores inside the X₁ compact, as illustrated by the schematics in FIGS. 3C and 6A, to form a composite material consisting of a p- or n-type semiconductor network provided by material X₁ and the second material Y₁.

Infilling the porous compact and conformally coating the all available surfaces of X₁ with this second material requires precise control of the nanoscale thickness and/or of the amount and uniform distribution of the deposited second material Y₁ on the surfaces of X₁. While atomic layer deposition (ALD) may often be the preferred technique for infilling and depositing the second material Y₁ on the surfaces of X₁, this infilling/depositing step can be performed by any available technique for thin film deposition, such as chemical vapor deposition (CVD), atomic layer deposition (ALD), electro-chemical deposition, chemical deposition from solution, etc., or infilling by infiltration by melt, from liquid solution, etc.

Finally, in Step IV (shown as Step 204 in FIG. 2 and schematically in FIG. 3D), the composite material formed in Step III is sintered to remove residual porosity from the composite material so as to form a solid having an intimately connected percolated p- or n-type network of X₁ and a separate network of Y₁ bonded to X₁, and having strong chemical bonds at all interfaces between materials X₁ and Y₁. While in some cases, the sintering process can result in a material having at least some residual porosity, the sintering process should be conducted in a manner that preserves the intended structure of the composite and maintains a p- or n-type semiconductor network X₁ that percolates charge within the solid semiconductor/insulator/dielectric material Y₁. The final result is a TE nanocomposite solid comprising at least one component at nanoscale such as that illustrated by the block schematic in FIGS. 3D and 6A and having a p- or n-type network formed by semiconductor material X₁ which percolates charge through a solid formed from semiconductor/insulator/dielectric material Y₁, where Y₁ provides efficient phonon scattering in the final TE nanocomposite material so as to reduce its thermal conductivity while maintaining or improving electrical transport properties provided by the p- or n-type material X₁.

This sintering step can occur either inside or outside the deposition chamber, so long as any transfer of the material outside the deposition chamber is preferred to be conducted in a controlled atmosphere and with air-free transfer so as to provide clean interfaces within the thermoelectric nanocomposite.

Before the sintering step the material after the deposition can be annealed in the deposition chamber or in a furnace to remove any unwanted species, and then can be sintered either inside or outside the deposition chamber or the furnace, with any transfer of the material outside the deposition chamber or the furnace is preferably being conducted in a controlled atmosphere and with air-free transfer so as to provide clean interfaces within the thermoelectric nanocomposite.

In a second embodiment of a process for making a TE nanocomposite material in accordance with the present invention, Steps I, II, and IV proceed as described above with respect to the first embodiment. In Step III of this second embodiment, the process of infilling and conformally coating all available surfaces inside the porous compact of material X₁ with a material Y₁ is interrupted while the compact retains an open porosity, and the step of infilling is repeated with a second metal/semiconductor/insulator/dielectric material Y₂, as illustrated by the block schematic shown in FIG. 6B.

Material Y₁ can coat all available surfaces partially or completely and material Y₂ can also coat all available surfaces partially or completely. Y₁ and Y₂ can be metal or semiconductor, or dielectric as long as their combination will suppress thermal transport in the final TE nanocomposite material.

In a third embodiment of the process for making a TE nanocomposite material in accordance with the present invention, Steps I and II also proceed as described above with respect to the first embodiment, but in this third embodiment, Step III of infilling and conformally coating all available surfaces inside the porous compact of material X₁ with a material Y₁ is interrupted while the compact retains an open porosity, with the steps of infilling being repeated with materials Y₂, Y₃, ..., Y_(N-1) and finally with material Y_(N), where materials Y₁, Y₂, Y₃,,... Y_(N) can be the same or different and can be any materials including semiconductors, metals, or insulators, with the choice of material(s) being determined by the desired properties and application of the final TE material.

Materials Y₁, Y₂, Y₃,,... Y_(N) can coat all available surfaces partially or completely.

In a fourth embodiment of a process for making a TE nanocomposite material in accordance with the present invention, aspects of which are illustrated in FIGS. 4A-4D and 6C, Steps I, II, and IV proceed as described above with respect to the first embodiment, except that instead of single-material particles or nanoparticles, the starting material comprises core/shell nanoparticles having a core of material X₁ and a shell of material X₂, as illustrated by the schematic in FIG. 4A.

The material X₂ shell can be a continuous film as shown in FIG. 4A or can be a discontinuous film comprising separate or non-separate islands, nanoparticles or nanocrystals which can percolate or not percolate on the surface of particle X₁. Material X₂ can be any material including semiconductors, metals and dielectrics depending on design of the TE material.

As illustrated in FIG. 4B, these X₁/X₂ core/shell nanoparticles are then formed into a porous green compact, and are then infilled with an material Y₁ suppressing heat transport as illustrated in FIGS. 4C and 6D, which is made to conformally coat, partially or completely, all available surfaces in the X₁/X₂ core/shell green compact.

The infilled compact is then sintered (step IV), as illustrated in FIG. 4D, to form a solid TE material in which the initial core/shell X₁/X₂ particles percolate through a Y₁ matrix, where Y₁ provides efficient phonon scattering in the final TE nanocomposite material so as to reduce its thermal conductivity while maintaining the electrical properties provided by p- or n-type materials X₁ or/and X₂.

In a fifth embodiment of a process for making a TE nanocomposite material in accordance with the present invention, the process of infilling and conformally coating all available surfaces inside the porous compact of material X₁/X₂ with a material Y₁ (semiconductor, metal, dielectric, etc.) is interrupted while the compact retains an open porosity and the step of infilling is repeated with a material Y₂, where materials Y₁ and Y₂ can be any materials including semiconductors, metals and dielectrics, with the choice of material(s) being determined by the desired properties and application of the final TE nanocomposite material. In an exemplary embodiment, material Y₁ can be ZrO₂ and material Y₂ can be ZnO.

In a sixth embodiment of a process for making a TE nanocomposite material in accordance with the present invention, the process of infilling and conformally coating all available surfaces inside the porous compact of the material X₁/X₂ is repeated with materials Y₁, Y₂, ..., Y_(N-1) while the compact retains an open porosity for the final infilling step with material Y_(N). Any one or more of materials Y₁, Y₂, Y₃,,...Y_(N) can be any materials including semiconductors, metals and dielectrics, with the choice of material(s) being determined by the desired properties and application of the final TE material.

In a seventh embodiment of a process for making a TE nanocomposite material in accordance with the present invention, the starting powder consists of core/shell nanoparticles having a structure X₁/X₂/.../X_(N) where material X₁ is the core, X₂ ... X_(N-1) are intermediate continuous or discontinuous layers, and X_(N) is the outer continuous or discontinuous shell, where X₁, X₂, ... X_(N) can be any materials including semiconductors, metals and dielectrics/insulators with at least one a semiconductor which has p- or n-type conductivity providing charge transport through the final TE material. The materials choices depend on the desired properties and application of the final TE material. For example, in some cases X_(N) can be very thin insulator, while X_(N-1) is semiconductor, with electrons from X_(N-1) tunneling through the X_(N) shell in the final product. In another example X_(N) can be islands of insulator letting parts X_(N-1) semiconductor of one particle to be in a direct contact with X_(N-1) semiconductor of another particle when they will be used to make porous compact. Yet, in another example X_(N) can be islands of a metal letting parts X_(N-1) semiconductor of one particle to be in a direct contact with X_(N-1) semiconductor of another particle when they will be used to make porous compact.

Thus, Step I in this seventh embodiment includes the step of making or selecting nanopowder consisting of X₁/X₂/.../X_(N) core/shell nanoparticles where material X_(N) is an outer shell.

In Step II of this seventh embodiment, the X₁/X₂/.../X_(N) core/shell nanopowder of the material is formed into a porous compact, creating a core/shell particle or nanoparticle network having an open porosity as described above with respect to the first embodiment.

In Step III of this seventh embodiment, the porous compact of the X₁/X₂/.../X_(N) material is infilled with a material Y₁. In some embodiments, Y₁ can be the same semiconductor material as X_(N), while in other embodiments, Y₁ can be a different semiconductor material or an insulator/dielectric material, so long as the addition of Y₁ increases phonon scattering in the final TE nanocomposite material so as to reduce its thermal conductivity while minimally affecting the electrical transport provided by p- or n-type X₁/X₂/.. ./X_(N). During this infilling step, the Y₁ material is made to conformally and completely or incompletely coat all available surfaces inside the compact formed from the X₁/X₂/.../X_(N) nanoparticles using any suitable technique, e.g., atomic layer deposition (ALD), with the result being a composite material comprising percolated X₁/X₂/.../X_(N) network and Y₁ network or domains. This composite material can then be sintered as in Step IV of the process described above to form the final TE material with charge transport provided by the initial X₁/X₂/.../X_(N) core/shell particles which percolate charge through a solid formed from X₁/X₂/.../X_(N) and material Y₁, where Y₁ increases phonon scattering in the final TE nanocomposite material so as to reduce its thermal conductivity while maintaining or improving electrical transport properties provided by the p- or n-type X₁/X₂/.../X_(N) structure.

In an eighth embodiment of a process for making a TE nanocomposite material in accordance with the present invention, the process of infilling and conformally partially or completely coating all available surfaces inside the porous compact of material X₁/X₂/.../X_(N) with a material Y₁ is interrupted while the compact retains an open porosity and the step of infilling is repeated with a material Y₂, where materials Y₁ and Y₂ can be any materials including semiconductors, metals and dielectrics, with the choice of material(s) being determined by the desired properties and application of the final TE nanocomposite material.

In a ninth embodiment of a process for making a TE nanocomposite material in accordance with the present invention, the process of infilling and conformally coating all available surfaces inside the porous compact of the material X₁/X₂/.../X_(N) in Step III is repeated with any materials Y₁, Y₂, ..., Y_(N-1) including metal/semiconductor/insulator/dielectric to have a multiple coatings on the surfaces while the compact retains an open porosity for the final infilling step with material Y_(N). Any Y₁, Y₂, ..., Y_(N) material can partially or completely coat all available surfaces .

In Step IV of this embodiment, the formed composite material is sintered in order to remove residual porosity and form a solid with intimately connected p- or n-type networks and having strong chemical bonds at all interfaces between materials. In some cases, the sintering process can result in a material having some porosity. The sintering process should be conducted in a manner that preserves the intended structure of the composite with p- or n-type semiconductor networks that percolate charge and results in a solid thermoelectric material that has no porosity while retaining the original nanoscale structure. The final TE nanocomposite material has a p- or n-type network formed by X₁/X₂/.../X_(N) which percolates through a solid formed from X₁/X₂/.../X_(N) and Y₁, Y₂, ..., Y_(N-1), Y_(N) materials, where the combination of Y₁, Y₂, ..., Y_(N-1), Y_(N) materials increase phonon scattering, thereby suppressing heat transport while maintaining or improving electronic transport provided by p- or n-type X₁/X₂/.../X_(N) material structure, e.g., by increasing the Seebeck coefficient of the TE material.

In some cases of the first three embodiments of a process for making a TE nanocomposite material described above, X₁ can be an insulator or a semiconductor and Y₁, Y₂, ..., Y_(N-1), Y_(N) can be any materials including semiconductors, metals and dielectrics/insulators, with at least one Y₁, Y₂, ..., Y_(N-1), Y_(N) material being a semiconductor which has p- or n-type conductivity providing charge transport through the final TE material, with X₁ enhancing phonon scattering in the final TE nanocomposite material to reduce its thermal conductivity while maintaining or improving electronic transport properties provided by p- or n-type Y₁, Y₂, ..., Y_(N-1), Y_(N) material.

In some cases of the fourth to ninth embodiments described above, materials X₁/.../X_(N) can be any materials such as a metal, a semiconductor or a dielectric leading to increased phonon scattering in the final TE nanocomposite material and reduction of its thermal conductivity, while maintaining or improving electronic transport properties provided by p- or n-type Y₁, Y₂, ..., Y_(N-1), Y_(N) material. Y₁... Y_(N) can be any material, with at least one is a p- or n-type semiconductor forming a percolated network of charge transport throughout the entire thermoelectric material.

In some embodiments, a TE nanocomposite material in accordance with the present invention can be made from one or more types of nanoparticles A, B, etc., at least one of them having n- or p-type conductivity, where the nanoparticles are sintered as described in the previous embodiments, to form a solid in which at least one type of the particles with n- or p-type conductivity forming a percolated network of charge transport throughout the entire material, while other nanoparticles suppress the heat transport.

In some such embodiments, one or more of nanoparticles A, B, etc., can be core/shell nanoparticles having one or more shell, and where the nanoparticles are sintered to form a solid having at least one percolation path or network with p- or n-type conductivity throughout the entire material. For example, FIGS. 5A-5D show shows exemplary steps for making TE nanocomposite material in the case of one core/shell nanoparticle A with one shell.

In all of these embodiments, a TE nanocomposite solid can be obtained, where the TE nanocomposite solid that contains interpenetrating three-dimensional p-type or n-type networks that percolate throughout the solid. Band-like transport of electrons or holes across the entire TE solid is ensured by sufficiently large cross-sectional areas of the conductive n-type or p-type channels; the former are achieved through the sintering process and through the heavy doping of the p-type or n-type networks.

These and other suitable configurations of TE materials would be readily understood to be possible by one skilled in the art are all deemed to be within the scope and spirit of the present invention.

Example

In this example, a thermoelectric nanocomposites made from p-type silicon (Si) nanopowder with aluminum oxide (Al₂O₃) and p-type silicon (Si) nanopowder with zirconium oxide (ZrO₂) made in accordance with the first embodiment are demonstrated.

Si nanopowder with p-type conductivity is made by high-energy milling in pure argon of Si bulk material doped with boron, having resistivity of about 0.001-0.005 ohm·cm. The resulting average crystallite size of the milled nanopowder is 30 nm.

The nanopowder is annealed at 450° C. for 2 hours in pure argon to remove moisture and other adsorbates from the powder surface.

After cleaning, the powder is transferred to a glovebox (GB) without exposure to air. Inside the GB the powder is compacted into a cylindrical shape 10 millimeters in diameter and 1 millimeter in height. The same procedure was repeated to make the second compact. Both compacts are transferred to a furnace without exposure to air and pre-sintered at 1000° C. for 5 minutes in pure argon. After pre-sintering, compacts have 45% porosity.

Following the pre-sintering, one compact is transferred to an ALD reactor in which an amorphous Al₂O₃ coating approximately 0.75 nm in thickness is deposited on all Si surfaces inside the pores. The Al₂O₃ coating is deposited via ALD at 180° C. using TMA as the Al precursor and water as the oxidant.

Following the pre-sintering, another compact is transferred to an ALD reactor in which a ZrO₂ coating approximately 0.75 nm in thickness is deposited on all Si surfaces inside the pores. The ZrO₂ coating is deposited via ALD at 180° C. using TDMAZ as the Zr precursor and water as the oxidant.

After the ALD step the formed composite material is sintered under a pressure of 1.0 GPa and temperature of 900° C. in order to remove residual porosity and form a solid with intimately connected p-type Si domains surrounded with Al₂O₃ or ZrO₂ and strong chemical bonds at all interfaces.

The resulting materials represents a thermoelectric nanocomposite solids with percolating p-type semiconductor and insulator efficiently scattering phonons and providing low thermal conductivity.

Thermal transport properties of these TE nanocomposite solids are compared with each other and with bulk Si and nanocrystalline Si. Plots of thermal conductivity are shown in FIG. 7 . Thermal conductivity of pure Si is substantially dropped in nanocrystalline Si when compared with bulk Si. It is a clear indication of enhanced phonon scattering on grain boundaries. Addition of very thin oxide films to the nanocomposites further reduced thermal conductivity (k) which leads to improved figure of merit ZT of thermoelectric material following the formula

$ZT = \frac{\sigma S^{2}T}{\kappa}.$

Thus, the present disclosure describes various embodiments of a thermoelectric nanocomposite material comprising p- or n-type semiconductor nanoparticles of a material X in an insulator/dielectric matrix of a material Y, where the insulator/dielectric material Y provides efficient phonon scattering so as to reduce the final TE material’s thermal conductivity while maintaining or improving the electrical transport properties provided by the p- or n-type material X.

Although particular embodiments, aspects, and features have been described and illustrated, one skilled in the art would readily appreciate that the invention described herein is not limited to only those embodiments, aspects, and features but also contemplates any and all modifications and alternative embodiments that are within the spirit and scope of the underlying invention described and claimed herein. The present application contemplates any and all modifications within the spirit and scope of the underlying invention described and claimed herein, and all such modifications and alternative embodiments are deemed to be within the scope and spirit of the present disclosure. 

What is claimed is:
 1. A method for forming a thermoelectric (TE) nanocomposite material, the method including steps of: making or selecting a powder comprising particles of a first semiconductor material X₁ having an n- or p-type conductivity; making a porous green compact consisting of an interconnected particle network from the powder of the first material X₁, the compact having an open porosity allowing permeation of the compact with gas or liquid; infilling and conformally coating all available surfaces inside the porous compact of the first material X₁ with at least one second material having a thermal conductivity lower than a thermal conductivity of the first material to form a composite material of the first and second materials, the second material Y₁ being a semiconductor or a dielectric/insulator material; and sintering the formed nanocomposite material to remove residual porosity from the composite material and form a nanocomposite solid material with intimately connected p- or n-type networks and having strong chemical bonds at all interfaces; wherein the nanocomposite solid material retains the nanostructure of the starting nanoparticles within the solid; wherein the nanocomposite solid material maintains a percolating p- or n-type semiconductor network of first material X₁ within the solid second material; and wherein the second material provides efficient phonon scattering so as to reduce a thermal conductivity of the nanocomposite material while maintaining or improving electrical properties of the first material.
 2. The method according to claim 1, wherein the powder is a nanopowder comprising nanoparticles having a particle size of about 1 nm to about 800 nm.
 3. The method according to claim 1, wherein the porous green compact is formed by pressing the nanopowder of the first material X₁.
 4. The method according to claim 1, wherein the porous green compact is formed by pre-sintering of the first material X₁.
 5. The method according to claim 1, wherein the step of infilling and conformally coating all available surfaces inside the porous compact of the first material X₁ with the second material is interrupted while the compact retains an open porosity and the step of infilling is repeated with an additional material Y₂.
 6. The method according to claim 5, wherein the process of infilling and conformally coating all available surfaces inside the porous compact of the first material X₁ includes N steps of infilling and coating all available surfaces inside the porous compact of material X₁ with materials Y₁, Y₂, ..., Y_(N) which are repeated while the compact retains an open porosity.
 7. The method according to claim 1, wherein the conformal coating is a continuous film of one or more of the Y₁, Y₂, ..., Y_(N) materials.
 8. The method according to claim 1, wherein the conformal coating is a discontinuous film of the Y₁, Y₂, ..., Y_(N) materials.
 9. The method according to claim 8, wherein the discontinuous film comprises nanoparticles or islands of one or more of the Y₁, Y₂, ..., Y_(N) materials.
 10. The method according to claim 6, wherein the N steps of infilling and conformally coating all available surfaces inside the porous compact of material X₁ with materials Y₁, Y₂, ...., Y_(N) are realized by means of atomic layer deposition.
 11. The method according to claim 1, wherein all processing steps are conducted in a controlled atmosphere with air-free transfer between steps to provide clean interfaces within the thermoelectric nanocomposite.
 12. The method according to claim 1, wherein the first material X₁ comprises nanoparticles of SnSe, Bi₂Te₃, a Bi-Te alloy, a BiSbTe alloy, Bi₂T₃/CdTe core/shell nanoparticles, a Zn-Sb alloy, Si, Ge, SiGe, Mg₂Si, SrTiO₃, NaCo₂O₄, Zn₄Sb₃, a Co-Sb alloy, or ZnO.
 13. The method according to claim 1, wherein the second material Y₁ comprises SiC, Al₂O₃, ZrO₂, HfO₂, SiO₂, Gd₂Zr₂O₇, and (Zr,Hf)₃Y₃O₁₂, Si₃N₄, A1N, ScN, MgF, CaF, ZnF, A1P, SiS₂, LiCl, NaCl, MgCl₂, or CaCl₂.
 14. The method according to claim 1, wherein the first material X₁ comprises nanoparticles having at least one length dimension of less than 3 nm. 